This image made from amateur video released by the Ugarit News and accessed Tuesday purports to show black smoke rising from buildings in Aleppo, Syria. United Nations observers say fighter jets are firing on anti-government rebels in Aleppo. The airstrikes come after President Bashar Assad issued a rare statement urging his armed forces to step up the fight against the rebels. (ASSOCIATED PRESS)

BEIRUT (AP) — Syrian President Bashar Assad urged his military Wednesday to boost its fight against rebels, but his written call to arms only deepened a mystery over his whereabouts two weeks after a bomb penetrated his inner circle.

Assad has not spoken publicly since the July 18 bombing killed four of his top security officials — including his brother-in-law — during a rebel assault on the capital, Damascus. The president’s low profile has raised questions about whether he fears for his personal safety as the civil war escalates dramatically.

The United States called the Syrian president a coward for marshaling his forces from the pages of the army’s official magazine.

“We think it’s cowardly, quite frankly, to have a man hiding out of sight, exhorting his armed forces to continue to slaughter the civilians of his own country,” said U.S. State Department spokesman Patrick Ventrell.

Sausan Ghosheh, the spokeswoman for the U.N. mission in Syria, said Wednesday that international observers witnessed warplanes firing in Aleppo, Syria’s largest city, where intense fighting has been raging for 12 days.

Speaking to reporters in Damascus, Ghosheh said the situation in Aleppo was dire.

“Yesterday, for the first time, our observers saw firing from a fighter aircraft. We also now have confirmation that the opposition is in a position of having heavy weapons, including tanks,” she said, adding that for civilians, there “is a shortage of food, fuel, water and gas.”

The U.N.’s World Food Program said it was sending enough emergency food aid for 28,000 people in the city of 3 million. The U.N. has estimated that some 200,000 residents have fled Aleppo.

On Wednesday, a Ukrainian military plane evacuated from Aleppo dozens of Ukrainians and Polish women with their children and in some cases Syrian husbands.

As the country delves further into chaos, there are mounting concerns about Syrian rebels carrying out atrocities against regime supporters.

A video posted online, which was impossible to verify independently, appeared to show rebels executing a man they identified as a member of the “shabiha,” or a pro-regime militiaman, in a hail of gunfire. Such developments pose a serious problem for the opposition, which has tried to claim the moral high ground against an authoritarian regime that has been accused of war crimes.

The conflict in Syria, which activists say has killed more than 19,000 people since March 2011, has drawn deep international condemnation. But world powers have few options to help beyond diplomacy — in part because of fears that any military intervention could make matters worse. Syria’s close ties to Iran and the Islamic militant group Hezbollah in Lebanon mean that the conflict has the potential to draw in the country’s neighbors.

Arab countries are pushing ahead with a symbolic U.N. General Assembly resolution that tells Assad to resign and turn over power to a transitional government. It also demands that the Syrian army stop its shelling and helicopter attacks and withdraw to its barracks.

A vote is set for Thursday morning.

The draft resolution takes a swipe at Russia and China by “deploring the Security Council failure” to act. Moscow and Beijing have used their veto in the Council three times to kill resolutions that might have opened the door to sanctions on Syria.

While the 193-member General Assembly has no legal mechanism for enforcing such a resolution, it can carry moral and symbolic power if a vote is overwhelming.

But the fighting has defied previous attempts at diplomacy. Residents of Aleppo have told The Associated Press over the past week that jet fighters have been strafing rebel positions. Activists have posted numerous videos on the Internet showing rebels commandeering regime tanks after seizing their bases.

Aleppo has been plagued with violence since mid-July, when rebels first attempted to take it over. The rebels have succeeded in holding several neighborhoods despite daily assaults by regime tanks, helicopters and warplanes.

Syria’s state news agency on Wednesday claimed several victories by government forces in Aleppo, especially in the hotly contested rebel bastion of Salaheddine. It said dozens of “terrorists” were killed, including some with African nationalities.

Rebels gave a different account, saying they had extended their control over the strategic city by taking two police stations.

Assad’s appeal to his armed forces, on the 67th anniversary of the Syrian army’s founding, appeared in the army’s magazine and was carried on the state news agency.

“Today you are invited to increase your readiness and willingness for the armed forces to be the shield, wall and fortress of our nation,” Assad said.

The regime has characterized the rebellion as the work of foreign terrorists, and Assad said that “internal agents” are collaborating with them.

“Our battle is against a multifaceted enemy with clear goals. This battle will determine the destiny of our people and the nation’s past, present and future,” he said.

The newly appointed defense minister, Gen. Fahd al-Freij, whose predecessor was killed in the July 18 bombing, echoed Assad’s words during a televised speech.

“The armed forces will pursue the remnants of these groups wherever they are and eliminate them, preserving the homeland from their evils and restoring peace and security to the country,” he said.

Assad’s only appearance since the July 18 bombing came in a brief taped segment on state TV as he swore in the new defense minister. But the clip had no audio, and it was unclear where it was shot.

Syria’s powerful military, which is vital to keeping Assad in power, has largely held together over the course of the uprising. The pace of defections has risen recently, however. Neighboring Turkey has reported that 28 generals have already crossed the border.

In recent weeks the military has unleashed heavy weapons against the increasingly bold rebels, who have brought the fight to the country’s two largest cities. The military managed to drive the rebels out of Damascus after they made stunning — but short-lived — advances there. Rebels claimed responsibility for the bomb that killed four top Assad aides.

Minor clashes around Damascus continue, however, and residents of the Christian neighborhood of Bab Touma in the old city of Damascus reported a half-hour gun battle early Wednesday.

There was also ongoing fighting in several other cities, including Homs in central Syria. Homs was bombarded by mortars, artillery and rockets, according to the Britain-based Syrian Observatory for Human Rights, which cites a network of sources on the ground.

Also Wednesday, Turkey launched a military drill just across the border from a Syrian town it claims is controlled by Kurdish rebels — a show of muscle aimed at Kurdish separatists pushing for autonomy within Turkey’s borders.

The Turkish government last week said Turkish Kurdish rebels have seized control of five towns along the border in collaboration with their Syrian counterparts. Turkey alleges that the Kurdish guerrillas they are fighting have taken advantage of the strife in Syria to take refuge there.

As the chaos deepens, there are mounting concerns that foreign jihadists and extremists are joining the fray. In a message that appeared in online jihadist forums, Syrian militant Abu Hussam al-Shami called on Muslims to come fight a jihad, or holy war, against the regime because it had committed atrocities against its own people.

Addressing the regime in the nearly 10-minute video, al-Shami said: “We will not be satisfied until we turn you into ashes like those under a cooking pot.”